Ijoer jul 2015 13

International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-4, July- 2015]

Heat Transfer and Flow Friction Characteristics of Solar Water Heater with Inserted Baffel Inside Tube U. K. Nayak1, Prof. (Dr.) S. C. Roy2, Prof. (Dr.) M K Paswan3, Dr. A. K. Gupta4 1,2

Mech .Engg. Deptt. BIT Sindri, Dhanbad-828123, Jharkhand, India, 3 Mech. Engg. Deptt. NIT Jamshedpur,India 4 Chem.. Engg. Deptt. BIT Sindri, Dhanbad-828123, Jharkhand, India

Abstract— Experimental investigation of friction factor and heat transfer characteristics of thermosyphone solar water heater with flat plate solar collector fitted with full length baffle of 5cm pitch have been presented. The flow regime is laminar for this study with the Reynolds number range 139 to 279. The experimental data obtained were compared with those obtained from plain tube data and 10cm pitch baffle. The effects of full length baffle inside the tube on heat transfer and friction factor were presented. The heat transfer coefficient for baffle tube with 5cm pitch is higher than that of plain tube and baffle tube with 10cm pitch for a given Reynolds number. The use of baffle created improved the performance of thermosyphone solar water heater.

Keywords— Augmentation, baffle creates, Heat Transfer, Reynolds Number, flat plate collector.

I.

INTRODUCTION

To combat from pollution due to fossil fuel burning Solar water heating has been identified as an important means. In India water heating has large potential because of abundant sunshine availability. In the rural areas of India, the most commonly used fuel is still the biomass. In winter time in India the northern side of the country, the water chills down below 0 oC in the peak winter. Warm water requires in house for daily work. Almost 35-40 L of warm water is needed for a person per day in the winter season. To get higher efficiency is solar water heater many designs have been investigated. For domestic purpose most commonly used solar water heater is natural circulation type wherein the hot water is circulated between collector and storage water tank. These types referred to as thermpsyphon systems and have been extensively investigated. For better use of heat, in thermosyphone solar water heater higher thermal conductive material should be used . The techniques to enhance heat transfer are related to several engineering applications also. The cost of energy and material in recent year is high has resulted in an sincere effort aimed to produce higher efficient thermosyphone solar water heater. The rate of heat transfer can be improved by introducing a disturbance in the fluid flow to break the hydrodynamic and thermal boundary layers.

Therefore, to acquire a desired heat transfer rate in an existing solar water heater at an economic pumping power, several techniques have been proposed by different researcher in recent years. In “HEAT TRANSFER AND FLOW FRICTION CHARACTERISTICS OF SOLAR WATER HEATER WITH INSERTED BAFFEL INSIDE TUBE" ,in IJMET, Volume 5, Issue 4, April (2014), pp. 16-22 with 10cm pitch of baffle is presented. Kumar and Prasad, presented a remarkable work on inserting twisted tapes in a serpentine solar collector. Their investigation is on the effect of the twistedtape geometry, different mass flow rates and intensity of solar radiation on thermal performance. Their observation was that heat losses were reduced for the lower value of the plate temperature and consequences was an increase on the thermal efficiency was observed. Hobbi and Siddiqui conducted an experiment to investigate the effect of several insert devices on the thermal performance of a flat-plate solar collector. Their studied for different passive heat enhancement devices like twisted strips, conical ridges and coil rings wires. They did not find appreciable difference in the heat transfer to the collector fluid and finally come to the conclusion that the applied passive methods based on the enhancement of viscous- produced turbulence were not effective in increasing heat transfer to the collector fluid. Sivashanmugam, investigated the heat transfer and friction factor characteristics of a circular tube fitted with right-left helical screw tape inserts of equal length, and unequal length of different twist ratios. The results analysed that the heat transfer augmentation for right-left helical screw tape inserts is higher than that for straight helical twist due to the effect of repeated both left and right movement of fluid during the flow through tube fitted with left-right twist tape provides better mixing in the radial direction. Rahimi , investigated the effect of three modified twisted tapes ( notched, perforated and jagged twisted tapes) inserts in the tube on the friction factor, Nusselt number and thermal performance through experiment and computational fluid dynamics (CFD).

Their results clears that the Nusselt number and thermal performance of the jagged insert were higher than those of the others inserts. Eiamsa-ard, gone through a comparative investigation of enhanced heat transfer and pressure loss by insertion of single twisted tape, full-length dual twisted tape and regularly-spaced dual twisted tape as swirl generators. The result reveals that all dual twisted tape with free spacing generates lower heat transfer enhancement in comparison with the full-length dual twisted tape. Work on twisted- tape started with insertion of twisted-tape in circular ducts ,later it is used in non circular duct also . The width of the metal strip of twisted-tape is kept equal to or less than the internal diameter of the duct according to the loose or tight fitting inside the tube. Reduction of tape width causes a decrease in the pressure drop and heat transfer Page | 33

International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-4, July- 2015]

rate. The twisted-tapes may be of full length of the duct, may be of the short length or may be used for a certain length of the duct. In the present work the baffle is created inside the tube

II.

NOMENCLATURE

T1 =

Temperature of inlet of solar collector box

T2 =

Temperature of outlet of solar collector box (

T3 =

Upper layer temperature of water tank (

)

T4 =

Lower layer temperature of water tank (

)

Ta =

Ambient Temperature (0C)

Tfluid =

Average temperature of inlet and outlet temperatures (

Tp =

Collector panel temperature (

Ac =

Solar collector area (m2)

FR =

collector heat removal factor

F’ =

collector efficiency factor

Cp =

Fluid’s specific heat (Kj/kg

fluid =

)

)

)

)

Fluid’s average convective heat transfer coefficient (w/m2

)

I=

Hourly solar radiation on the flat- panel solar collector (w/m2)

Lc =

Length of the collector panel channel (m)

L=

Length of the circular tubes (mm)

Q=

Rate of Heat transfer (kj)

Qaccumulated = Accumulated heat (kj) Qloss =

Heat transfer rate (kj/sec)

ƒ=

Friction factor.

∆p=

Pressure drop (Pa).

U=

Average Velocity of water flow (m/s).

ṁ=

Mass flow rate of water (kg/s).

px =

pitch of the baffle III.

EXPERIMENTAL SETUP

The schematic diagram of the natural circulation solar water heater is shown in Figure-1. The system consists of a solar flat plate collector, storage tank and connecting pipes. The absorber plate of the solar collector is of Aluminium, and six row of pipe length160cm and 2.54cm in diameter. Headers are in the two opposite sides of the box to maintain good contacts with the pipes. In the experiment 1600 mm long and diameter of 25.4 mm iron tubes are used. The pipes are placed parallel to each other and welded at both ends to the header. The front surface of the box is then covered with 4 mm thick clear plain glass and the overall dimension of flat-plate solar collector is 1380mmx750mmx250mm and the effective glazing area is 1537inch2. Same dimension is selected for both the case, but in the baffled pipe 10cm of pitch is selected. The solar collector along with its water tank was installed at various angles (20.5°, 18.2°, 15.4°, 15.7°, 14.3°, 15.3°, 17.6°, 16.5°, 15.2°) on each day of observation. The whole unit was oriented in the South .The Pyranometer measuring short wave radiation was connected at same slope to as the collector to read radiation flux(W/m2) on the inclined surface (as shown in figure).this pyranometer output reading was converted into a heat flux using the celebration relation (1mV= 129W/m2) provided by the manufacturer. Readings were obtained for one inlet water temperature value (T1) and one outlet water temperature value (T2). in addition two readings of the temperature of water tank on top and bottom(T3) and (T4) of the water tank another thermocouple used to measured the absorber plate temperature (Tp). In addition, a thermocouple was also connected to measure and ambient temperature (Tamb).These six readings were made through the use of J-type thermocouple. The Page | 34

International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-4, July- 2015]

thermocouples were connected six channels of thermocouple amplifier .Once the unit was connected, it was left to run for about 2 days before the recorded measurements are taken, in order to overcome the initial transient’s effect and to conform reliable operation unit. Then the experiment was run for period of 5 days on Al- sheet .Experimental readings were taken in plain and baffle tube. Baffling was created to break boundary layers inside the tube and proper mixing of the fluid.

FIG-1: SCHEMATIC DIAGRAM OF EXPERIMENTAL SETUP IV.

DATA REDUCTION

Density difference of the fluid created by the temperature gradients causes the fluid being heated and delivered to storage tank. This type of fluid flow is usually termed as the natural or free convection. The temperature distribution, heat energy , collector efficiency and friction factor were calculated by using the equations (1) to (9) which were derived from [8], [9], [11] and [10].

The relationship between the mass flow rate (m) and the temperature , the equation for heat energy which is used for water heating (Qa), as:

Qa = AC FR [ Iατ – UL ( Tf – Ta )]

(1)

The heat energy is converted into thermal energy of water in the pipes, as:

Qa = mCp (T1-T2)

(2)

Then

mCp (T1 - T2) = Ac FR [ Iατ − UL ( Tf- Ta)]

(3)

Therefore (T1 - T2) = (Ac FR / m Cp) [Iατ − UL (Tf - Ta)] FR =mCp/AcUL [1- exp (ULF’Ac/mcP)]

(4) (5)

Then the collector efficiency is obtained by using the relation, η = Qa /AcI

(6)

Substitution of Eqs. (2) and (4) in Eq. (5) yields, η = FR [ατ – UL (Tf– Ta) / I]

(7)

Since FR, ατ & UL are constant, therefore, η α [(Tf– Ta) / I]

(8)

Page | 35

International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-4, July- 2015]

For calculation of friction factor , The Darcy-Weisbach equation:

ƒ=

=

(9)

FIG. 2 GRAPH BETWEEN TIME AND INTENSITY

FIG.- 3 GRAPH BETWEEN TIME AND EFFICIENCY

FIG.- 4 GRAPH BETWEEN TIME AND OUTLET TEMPERATURE Page | 36

International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-4, July- 2015]

FIG.- 5 GRAPH BETWEEN TIME AND FRICTION FACTOR V.

RESULTS AND DISCUSSIONS

The Natural circulation solar water heater was examined in the month of December-January, 2012-13 at intervals of quarter an hour between 9.30 hours and 13.30 hours. The incident solar radiation intensity was measured using pyranometer. The water inlet and outlet temperatures for the collector as well as ambient air were measured by thermocouple. The mass flow rate of the system was measured by rotameter. The collector efficiency of the system was calculated using Eq. (1) to Eq. (9).

The quarterly variation of the solar intensity, collector efficiency and collector water outlet temperatures and Friction factor are shown in Figures 2, 3, 4 and 5. The solar intensity is increasing from 9.30 hours to 12.30 hours, reaching a maximum value of 621.12 W/m2 at 12.30 hour for baffled tube with 5cm pitch,596.98w/m2 for tube with 10cm pitch at 12.45hours and for plain tube it was486.33W/m2 at 12.15 hours in Figure-2. The collector efficiency is also compared with three different cases. The collector efficiency at 12.45 hour is 46.57% for baffled tubes with 5cm pitch, 44.5% efficiency of tube with 10cm pitch at 12.45hours and 31.49% for plain tubes at the same time. The maximum efficiency is observed at the time 12.45 hour in all the cases. The collector efficiency decreases after 12.45 hour till further readings were taken in the same day. The collector efficiency is shown in Figure-3. The curve shows that the maximum efficiency is at 12.45 hour in all three cases.

The collector outlet temperatures are shown in Figure-4. The outlet temperatures at 9.45 hour is 18˚C for plain tube and for the baffled tube with 5cm pitch it was 21˚C and for 10cm pitch it was 240C. The maximum outlet temperatures were recorded at 12.45 hour for all the cases. The outlet temperature fall after 12.45 hour till further record. The Friction factor curve against time is shown in the figure- 5. According to the curve it can be say that the value is falling continuously and the friction factor value at 12.45hours for baffled tube with 5cm pitch is 0.0366 and for tube with 10cm pitch it is 0.03265 and for the plain tube it was 0.03715.

VI.

CONCLUSION



The collector efficiency is increases for some time and after which decreases, the maximum value is obtained at 12.45hours in all the cases.



From the graph it can be say that the efficiency for baffled tube with lower pitch is higher in comparison to plain tube and with higher pitch length.



Outlet temperature of the fluid is higher for baffled tube with 5cm pitch is higher comparison to other two cases found maximum at 12.45hours.



Friction factor in the baffled tube with lower pitch length at higher outlet temperature is lower in comparison to baffled with higher pitch length but higher to plain tube, it means fluid mixing is better.

REFERENCES [1] A. Kumar, B. N. Prasad. Investigation of twisted tape inserted solar water heaters—heat transfer, friction factor and thermal performance results. Renewable Energy, 19 (3), 2000, pp. 379-398.

[2] A. Hobbi, K. Siddiqui. Experimental study on the effect of heat transfer enhancement devices in flat-plate solar collectors. Int. J. of Heat and Mass Transfer 52, 2009, pp. 4650–4658.

[3] Sivashanmugam, P., Nagarajan, P.K., Suresh, S., Experimental Studies on Heat Transfer And Friction Factor Characteristics Of Turbulent Flow Through a Circular Tube Fitted with Right and Left Helical Screw-Tape Inserts, Chem. Eng. Commun., vol.195, 2008, pp. 977– 987.

Page | 37

International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-4, July- 2015]

[4] Rahimi, M., Shabanian, S.R., Alsairafi, A.A., Experimental and CFD studies on heat transfer and friction factor characteristics of a tube equipped with modified twisted tape inserts, Chem. Eng. Process, Vol.48, 2009, pp. 762– 770.

[5] S. Eiamsa-ard, Chinaruk Thianpong , Petpices Eiamsa- ard and Pongjet Promvonge, Thermal characteristics in a heat exchanger tube fitted with dual twisted tape elements in tandem, International Communications in Heat and Mass Transfer Vol. 37 (2010), PP.39–46.

[6] C.Nithiyesh Kumar, P.Murugesan, Review on Twisted Tapes Heat Transfer Enhancement, International Journal Of Scientific & Engineering Research, Volume 3, Issue 4, April-2012 .

[7] P. Sivakumar1, W. Christraj2, M. Sridharan1 and N. Jayamalathi, PERFORMANCE IMPROVEMENT STUDY OF SOLAR WATER HEATING SYSTEM, ARPN Journal of Engineering and Applied Sciences , VOL. 7, NO. 1, JANUARY 2012 [8] Gupta H.P. and Garg H.P. 1968. System designs in solar water heaters with natural circulation. Solar Energy. pp. 12-163.

[9] Rai. G.D. 2005. Solar Energy Utilization. Khanna Publishers. pp. 102-104 and 135-145.

[10] Sukhatme S.P. and Nayak J.K. 2008. Solar Energy, Principles of Thermal Collection and Storage. TMH [11] Soteris A. Kalogirou. 2004. Solar thermal collectors and applications Progress. Energy and Combustion Sciences. 30: 231-295.

[12] Nayak U.K ,Roy S.C. Paswan M.K.,Gupta A. K.2014 HEAT TRANSFER AND FLOW FRICTION CHARACTERISTICS OF SOLAR WATER HEATER WITH INSERTED BAFFEL INSIDE TUBE" ,in IJMET, Volume 5, Issue 4, April (2014), pp. 16-22

Page | 38